1. Field of the Invention
The present invention relates, in general, to a method for preparing polyester film suitable as a base film for magnetic tape and, more particularly, to an improvement in productivity, abrasion resistance and slip properties along with the method.
2. Description of the Prior Art
Polyesters industrially prepared, especially, ones obtained from aromatic dicarboxylic acid and ethylene glycol are extensively used for fibers, films and mold articles by virtue of superior mechanical, physical and chemical properties.
Industrial methods for the preparation of polyesters are typically exemplified by a direct esterification process or ester-interchange process. In the direct esterification, terephthalic acids and ethylene glycols are heated to reaction temperatures ranging from 200 to 280.degree. C. under atmosphere or a pressure, to directly esterify them. On the other hand, the ester-interchange process comprises two steps: first, methyl terephthalates and ethylene glycols are reacted in the presence of catalysts at reaction temperatures ranging from 140 to 240.degree. C., to synthesize prepolymers, that is, bis(beta-hydroxyethyl) terephthalate and/or low molecular weight polymers thereof (hereinafter "esterified compounds"); and, in the second step, the synthesized, esterified compounds are polycondensed in the presence of polymerization catalysts at higher reaction temperatures ranging from 260 to 300.degree. C. in vacuo, so as to obtain high molecular weight polymers.
Polyester films are generally superior in thermal resistance, mechanical properties, dimensional stability, and electric insulation and useful for a plurality of purposes including industrial field, such as magnetic recording film, photofilm, insulating substance and film for deposition, and agricultural field. Also, their consumption amount as a package use for food or other materials has sharply increased because of good chemical properties, transparency, odor preservativity, water resistance and gas barrier.
Slip property and abrasion resistance of polyester film as well as its composition significantly affect the workability in manufacture procedure of polyester film. Particularly, when base films for magnetic tape are manufactured from polyester, these properties are very important to the productivity. For example, if too much diethylene glycol and terminal carboxyl group are contained in a composition for polyester film, thermal stability is degraded, which forces the wiping cycle of extrusion nozzle to be short and thereby lowers productivity. In addition, when melt-extruded sheets are chilled on the surface of a rotary cooling drum, it is difficult to improve the productivity through casting speed if the sheets do not stick fast to the drum. The films thus obtained are nonuniform in thickness, resulting in the production of a magnetic tape with poor electromagnetic characteristics.
And, lack of slip property and abrasion resistance in polyester base film increases the friction between a coating roll and the film upon producing magnetic tapes, leading to generation of crumples and scratches therein. In addition, powder or coating is discontinued by the abrasion of film, which results in drop out of magnetic record.
Recently, it has been indicated that the scratches generated by the contact between the magnetic face and non-magnetic faces at the winding step after the steps of coating of magnetic materials and calendaring degrades the quality of video tapes. Consequently, the polyester film composition which allows the sheets to adhere fast to a drum can improve the productivity and quality of the polyester film and better abrasion resistance and slip property of polyester base film permits the production of higher quality magnetic tape.
Fundamental requirements to polyester film are largely divided into two: workability necessary for production of film, such as chemical, physical and electrical properties of polyester; and particle properties which determines the surface properties of film, such as abrasion resistance and slip properties.
Typically, polyester film is prepared by melt-extruding polyester film composition into a sheet, chilling it on a cooling drum to give an amorphous sheet, and drawing and heat fixing to produce a biaxially oriented film. In order to increase the uniformity of the film thickness and the casting speed, the sheet melt-extruded from extrusion nozzle should adhere fast to the cooling drum when chilling. So-called electrostatic adhesion casting method is well known to be effective for improving the close adhesion between the sheet and the drum, in which static electricity is generated on the surface of an uncured sheet through application of high voltage into a wire electrode set between the extrusion nozzle and the rotary cooling drum and the sheet is chilled on the cooling drum. Uniformity in film thickness is one of the most important factors determining the quality of polyester film. Since the productivity of film directly depends on the casting speed, productivity is improved by increasing the casting speed, which is accomplished through electrostatic adhesion. Many charges on surface of the sheet can improve the electrostatic adhesion. It is known that lowering the specific resistance of polyester composition through modification is effective for the generation of many charges on the surface of the sheet. The specific resistance of polyester composition is determined by the kind, state and quantity of metallic components added in the composition. Generally, the specific resistance of polyester composition is lowered as much as the metal compound does not lose from the melt polyester composition by phosphorous compounds, that is, the metal compound is not deposited from the polyester. Accordingly, in order to lower the specific resistance of polyester composition, the amount of lost metal compound has been reduced, or alkali or alkali earth metal compounds have further been added after esterification or ester interchange reaction.
Such methods can reduce the specific resistance of polyester materials into a certain level at which the electrostatic adhesion can be improved. Recently, film thickness has become further limited, and at the same time, increased productivity has become a requirement higher productivity. However, conventional polyester compositions cannot allow quality film to be produced at high casting speeds. Further, the research directed only to the improvement of electrostatic adhesion may include a danger that metal compounds added deleteriously affect the physical and chemical properties of the polyester composition. Consequently, it is important to improve the electrostatic adhesion without degradation of polyester composition.
Fundamentally, polyester films must have slip property and abrasion resistance. These properties are particularly important in the case that polyester films are used as base films for magnetic tape. To provide polyester film with slip property, fine protrusions have been formed by two methods.
One of these methods, known as "outer particle addition method", comprises adding inactive inorganic particles or crosslinked polymers at any stage of the polymerization process, to generate protrusions or depressions on at least one surface of the final film, as disclosed in Japanese Pat. Pub. Laid-Open No. Sho. 52-78953 and Sho. 64-48836 and Pub. No. Sho. 55-41648, Korean Pat. Pub. No. 91-8997, U.S. Pat. No. 3,821,156 and European Pat. No. 536,602A. The other, known as "internal particle deposition method", is also to form protrusions or depressions on at least one surface of the final film by depositing the product of the reaction between ester interchange reaction catalyst and dicarboxylic acid, phosphorous compound or linear oligomer during the production of polyethylene terephthalate, as disclosed in Japanese Pat. Pub. No. sho 55-20496, sho 49-13234 and sho 50-6493 and Pub. Laid-open No. 60-35022, U.S. Pat. No. 5,006,589 and 4,138,386.
Interned particle deposition method allows superior abrasion resistance but external particle addition method does not. In the latter case, the inactive particles are surface-treated or peculiar particles are used. For instance, colloidal silica particles are modified with glycol as disclosed in Japanese Pat. Pub. Laid-Open Nos. Sho. 63-221158 and Sho. 63-280763 and with coupling agent as in Japanese Pat. Pub. Laid-Open No. Sho. 63-312345. Also, it is reported in Japanese Pat. Pub. Laid-Open No. 62-235353 that the surface of calcium carbonate particles is treated with a phosphorous compound.
The difference between the two methods are further described as follows.
The internal particle deposition method is advantageous in many aspects in that facility investment is low, dispersing agent is not necessary and the film is superior in abrasion resistance. In addition, there are little voids because of the high affinity of the internal particles for polyester, so that highly transparent polyester films with little rupture at film-forming step can be obtained. However, internal particle deposition method has many disadvantages. For example, rough macroparticles are easily formed. Further, it is limited to control the deposition amount between particles, particularly, the content and size of the particles. Furthermore, the polymerization procedure of the internal particle deposition method must be strictly controlled for uniformity in diameter and distribution of particles and for repetitive results.
In contrast with the internal particle deposition method, the external particle addition method requires grinding, classifying and dispersing facilities. A dispersing agent is necessary to prevent coagulation of particles, which has significant deleterious influence on the thermal resistance and electrical properties of the film. Further, the film produced is short of abrasion resistance and defective in affinity between the external particles and polyester, so that voids occur at film-forming step, leading to rupture or opaqueness of film.
At present, with the advance of high performance and productivity, the conditions for operating magnetic tape films become more severe. Under these conditions, the polyester films which are prepared from the compositions of the conventional methods cannot be considered to have sufficient abrasion resistance.